The operation of axial flow gas turbine engines involves the delivery of compressed air to the combustion section of the engine where fuel is added to the air and ignited, and thereafter delivered to the turbine section of the engine where a portion of the energy generated by the combustion process is extracted by a turbine to drive the engine compressor. Accordingly, the efficiency of gas turbine engines is dependent in part on the ability to minimize leakage of compressed air between the turbine blades and a shroud that circumscribes the turbine.
To minimize the radial gap between the turbine blade tips and the shroud, turbine blades often undergo a final grind such that the turbine assembly closely matches its shroud diameter. As a result, some degree of rubbing with the shroud typically occurs during the initial operation of the engine due to manufacturing tolerances, differing rates of thermal expansion and dynamic effects. However, rubbing contact between the blade tips and shroud tends to spall the tips, which further increases the radial blade-shroud gap and shortens the useful life of the blade. As such, it is well known in the art to form a dynamic seal between the rotor blades and the shroud by forming an abrasive ("squealer") tip on the end of the turbine blades. Prior art abrasive tips have often entailed abrasive particles dispersed in an oxidation-resistant metallic matrix, as evidenced by U.S. Pat. No. 4,169,020 to Stalker et al., assigned to the assignee of this invention. During initial operation of the turbine, the abrasive tip abrades a groove in the shroud as a result of numerous "rub encounters" between the abrasive tip and the shroud. The groove, in cooperation with the blade tips as they partially extend into the groove, forms a virtual seal between the blade tips and the shroud. The seal reduces the amount of gases that can bypass the blades, and thereby improves the efficiency of the turbine engine.
Much emphasis has been placed on developing suitable combinations of metal matrix materials with abrasive particles, as well as methods for their manufacture. Generally, material combinations have been developed to be capable of abrading metallic shroud materials, such as Bradelloy, CoNiCrAlY and iron, nickel and cobalt-base superalloys, while exhibiting minimal reactivity between the abrasive and matrix materials. In addition, suitable metal matrix materials must exhibit acceptable environmental resistance (i.e., oxidation and hot corrosion resistance) to the operating environment of a gas turbine engine. With these requirements, the prior art has proposed various metal matrix materials, including nickel-base alloys, and various abrasive materials, including nitrides such as BORAZON (cubic boron nitride), carbides, and oxides such as aluminum oxide (alumina). Though widely used, BORAZON will not survive the typical turbine engine environment. As such, blade tips formed with this material are no longer abrasive after an initial engine "green run." Abrasive materials capable of surviving in the hostile environment of a gas turbine engine are often preferred to protect the blade tips during rub encounters that occur during in-service operation of the engine.
As engine performance has pushed gas path temperatures up, the use of shrouds formed of ceramic materials has increased. However, ceramic shrouds are more difficult to abrade than prior art metallic shrouds. One solution has been to reduce the shroud density by increasing the porosity of the ceramic material, though a drawback is that such shrouds are more susceptible to gas stream erosion or gas-borne particulate erosion than are shrouds formed from denser ceramic materials. Another difficulty encountered with the use of ceramic shrouds is that the ceramic shroud material is more abrasive to the metal matrix material of the abrasive blade tip, causing higher wear rates for the abrasive particles. In addition, ceramic shrouds tend to sustain significantly higher surface temperatures, leading to melting or substantial strength loss of the metal matrix material. In response to these issues, the prior art has proposed the deposition of abrasive particles in the form of a blade tip coating, such as by plasma spraying. However, ceramic coatings of this type have been found to be prone to spallation.
Thus, it would be desirable to provide an abrasive blade tip material that is compatible with a ceramic shroud, and that can survive numerous rub encounters with a ceramic shroud, while also exhibiting suitable environmental and spall resistance within the hostile operating environment of a gas turbine engine.